Crude oil derived and gas-to-liquids diesel fuel blends

ABSTRACT

The invention provides a method for the production of a diesel fuel composition, said method using Gas-to-Liquid (GTL) diesel fuel as a blend component together with at least a crude oil derived diesel fuel, said method producing a diesel fuel composition which, when combusted in an engine, has reduced NOx and soot emissions. The invention further provides a diesel fuel composition and a method of reducing NOx and soot emissions from a Cl engine.

This application claims the benefit of priority of South African Provisional Application No. ZA 2004/3179, which was filed on Apr. 28, 2004, and which application is hereby incorporated by reference.

FIELD

This application relates to crude oil derived and Gas-To-Liquids (GTL) diesel fuel blends.

BACKGROUND

Synthetic fuels such as GTL (Gas To Liquids) diesel fuel have seen a significant rise in interest in recent years. They are considered to be extremely clean fuels, with negligible sulfur and aromatics, and are odor-free and have a cetane number of >70.

The GTL diesel fuel used in the examples in this patent specification was manufactured by means of the Sasol Slurry Phase Distillate (Sasol SPD™) process, which consists of three process steps, as depicted schematically in FIG. 1.

In the first step an auto-thermal reforming process is used to convert the natural gas into the synthesis gas, a mixture of CO and H₂. In a second step the synthesis gas is converted into a so-called syncrude containing predominantly paraffinic hydrocarbons, by a Fischer-Tropsch process. This syncrude is primarily in the form of waxes and distillates, which are further refined in a third, product upgrading step by means of mild hydro-processing, in order to produce products that meet commercial fuel specifications, such as diesel fuel and kerosene.

SUMMARY

The invention provides a diesel fuel composition comprising both crude oil derived diesel fuel, which crude oil derived diesel fuel has a density at 15 deg C. below 0.85 kg/l, a sulphur content of less than 10 mg/kg, a polyaromatics content of below 5 mass %, and a cetane number from 51 to 60, and Gas-to-Liquids (GTL) diesel fuel, which GTL diesel has a density at 15 deg C. of below 0.78 kg/l, a sulphur content of less than 1 mg/kg, polyaromatics below 0.1 mass %, and a cetane number above 65, in a volumetric ratio range of from 1:99 to 99:1 and with a molar H:C ratio of between 1.8:1 and 2.1:1.

The diesel fuel composition may have less than 10 mg/kg sulphur.

The diesel fuel composition may have less than 5 mass % polycyclic aromatics.

The crude oil derived diesel fuel may be a fuel meeting the EN590 specification.

The volumetric ratio range may be from 1:9 to 9:1.

The volumetric ratio range may be from 1:5 to 5:1.

The molar H:C ratio may be from 1.85:1 and 2.05:1.

The molar H:C ratio may be from 1.9:1 and 2.00:1.

The diesel fuel composition may have an ASTM D86 10% distillation temperature of from 180° C. to 220° C.

The ASTM D86 10% distillation temperature may be from 200° C. to 215° C.

The diesel fuel composition may have a flash point of between 60° C. and 80° C., typically from 65° C. to 78° C.

The diesel fuel composition may have a density at 15° C. of from 0.77 kg/l to 0.84 kg/l.

The diesel fuel composition may have a density at 15° C. of from about 0.8 kg/l to about 0.82 kg/l.

The diesel fuel composition may have a lower heating value of from 42 500 kJ/kg to 43 800 kJ/kg, usually from 43 100 kJ/kg to 43 600 kJ/kg, typically from 43 200 kJ/kg to 43 500 kJ/kg.

Use of Gas-to-Liquid diesel fuel as a blend component for a diesel fuel composition, which, when combusted in an engine, has reduced NOx and soot emissions, which composition comprises both crude oil derived diesel fuel meeting the European EN590 specification for sulphur-free diesel fuel (designated EU diesel), and the Gas-to-Liquids (GTL) diesel fuel, wherein the crude oil derived diesel fuel to Gas-to-Liquid diesel volumetric blend ratio ranges from 1:99 to 99:1 and the composition has a molar H:C ratio of between 1.8:1 and 2.1:1.

Reductions in both NOx and soot emissions may be obtained which are greater than indicated by the blending ratio of the GTL diesel in the crude oil derived diesel fuel.

Thus, more than 70% of the reduction in both NOx and soot emissions which may be be obtained with neat GTL diesel fuel, may be obtained with a 1:1 GTL:Crude derived diesel ratio.

More than 40% of the reduction in both NOx and soot emissions which may be obtained with neat GTL diesel, may be obtained with a 1:4 GTL:Crude derived diesel ratio.

However, in some embodiments the reduction in NOx emissions may be less than the reduction in soot emissions, and vice versa.

In some embodiments, the reduction in NOx may be minimal, however, the NOx will be reduced by the use of GTL diesel in accordance with the invention.

The properties of the composition and the blending ratios of the components are as described above for the composition.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanying FIGS. 1 to 6.

WRITTEN DESCRIPTION

The effect of GTL diesel fuel blends on exhaust emissions and engine performance has been studied. EU diesel fuel was used as a reference fuel, in addition to being the base stock for the blends. The properties of test fuels used in the investigation are shown in Table 1. TABLE 1 Properties of the fuels investigated in this study. EU 2005 GTL EU50 EU80 European 100% GTL 50:50 Blend 80:20 Blend sulphur-free Property Units diesel fuel EU:GTL EU:GTL diesel fuel Density @ 15° C. kg/l 0.768 0.802 0.821 0.836 Density @ 20° C. kg/l 0.765 0.798 0.817 0.832 Cetane Number 71 62 58 54 Total Sulphur mg/kg <1 4 6 7 D86 Distillation IBP ° C. 169 157 174 193  5% ° C. 180 193 204 214 10% ° C. 187 201 212 221 20% ° C. 200 215 225 233 30% ° C. 219 231 240 248 40% ° C. 235 248 256 264 50% ° C. 251 264 270 277 60% ° C. 267 277 282 287 70% ° C. 283 291 294 299 80% ° C. 297 305 307 313 90% ° C. 312 322 324 332 95% ° C. 321 337 339 354 FBP ° C. 329 346 350 360 Flash Point ° C. 59 66 76 82 Kinematic Viscosity @ 40° C. mm²/s 1.97 2.54 2.79 2.95 CFPP ° C. −19 −18 −17 −17 Cloud Point ° C. −18 −17 −15 −14 Total Aromatics* % m/m 0.1 13.5 21.5 26.8 Bi- and Polycyclic aromatics* % m/m 0.0 2.3 3.7 4.6 Hydrogen Content* % m/m 15.0 14.3 13.8 13.5 H/C ratio (molar)* — 2.10 1.98 1.91 1.86 Lower Heating Value* MJ/kg 43.8 43.5 43.2 43.1 HFRR Wear Scar Diameter μm 370 <400 <400 394 *Values for blends calculated according to blending ratio

Dynamometer tests were conducted with a Mercedes Benz™ E220 CDI vehicle, using the New European Driving Cycle (NEDC) emission test, and without any changes to the basic EU3 emission level engine calibration or engine hardware. The vehicle was tested with its standard calibration without any adaptation, with EU diesel, the 1:1 blend and for the neat GTL fuel. The relevant test vehicle data are shown in Table 2. TABLE 2 Test vehicle and engine data Vehicle designation Mercedes E 220 CDI Limousine Model year 2003 Transmission 6-speed manual gearbox Gross vehicle mass 2145 kg Engine designation MB OM646, EU3 emission level Displacement, 2, 2 L, in-line 4 cylinder, 4 valves per configuration cylinder Compression ratio 18:1 Fuel management Common rail fuel injection (peak pressure 1 600 bar) Air management Turbocharged (VNT), intercooled Emission control Cooled EGR, inlet swirl control, close coupled and underfloor oxidation catalysts Rated torque 340 Nm at 2 000 rev/min Rated power 110 kW at 4 200 rev/min

The results of the unadapted vehicle emission tests are depicted in FIG. 2 for the EU diesel, EU50, and GTL diesel fuel. The averaged results for the test runs are presented as the percentages relative to the EU diesel reference fuel. FC indicates the volumetric fuel consumption.

For neat GTL diesel fuel, an unexpectedly high reduction of >90% for HC and CO emissions was observed. The CO and HC reductions for the 50% blend scale roughly with the blending ratio. The NOx emissions were reduced marginally, with the 50% blend again showing about half the reduction of the neat GTL diesel fuel. The same applies for the HC+NOx data.

PM emissions were reduced by up to 30% with the GTL diesel. Surprisingly, a strong non-linear characteristic was evident with the 50% blend (EU50), which showed a reduction of approximately 22%.

The potential for further emission reductions with the test fuels, and including the optimisation of a limited number of software parameters in the Engine Control Unit (ECU) of the engine was then investigated. For this purpose, an engine mounted on a test bench was used. Steady state test runs were carried out at five operating points characteristic for NEDC emission test cycle. The software parameters investigated were the Exhaust Gas Recirculation (EGR) rate, the start of pilot injection (SOPI) and the start of main injection (SOMI). The five operating points are shown in Table 3 TABLE 3 Steady state engine test points chosen to reflect NEDC characteristics. Engine Test Engine Speed bmep Power Point (rev/min) (bar) (kW) Description 1 1 000 0 0 Pseudo Idle 2 1 600 3.3 9 Characteristic operating 3 2 000 2 7 points for the NEDC 4 2 000 5 18 emission test 5 2 800 4 20

FIG. 3 shows two examples of results obtained from the steady state test bench work. The figure depicts representative data for the effect of GTL diesel fuel and its blends on the soot—NOx trade-off characterisitc at two operating points, namely 1 600 rev/min and 3,3 bar bmep (brake mean effective pressure), and 2 000 rev/min and 5 bar bmep. In this case, the EGR rate was varied, while the SOPI and SOMI were kept constant and equal to the reference values. Soot emission levels were calculated from exhaust smoke levels determined by FSN (Filter Smoke Number) measurements.

It is evident that GTL diesel offers a significant reduction in terms of both soot emissions and NOx for all the EGR rates tested. The soot emission increase for decreasing NOx values follows the expected pattern, and enables a wide range of possible alternative software calibrations. Surprisingly, the strong non-linear behavior of the EU50 blend is again evident—this fuel exhibits almost the same benefits as neat GTL diesel fuel.

A design of experiments (DOE) method was used to numerically optimize the three software parameters simultaneously. The DOE predictions were verified by actual experiments, and an example of the results of the simultaneous optimisation of all three calibration parameters at each of the engine operating points is shown in FIG. 4. In this case the optimisation has been performed to minimise NOx emissions with the GTL diesel fuel. Reductions of between 30% and 75% were obtained, without compromising the other emissions, when compared to the EU diesel.

The measured data at the five steady-state test points was used to predict the emissions over the NEDC test cycle. Empirical factors were used to account for the differences between the steady-state and transient engine operation. All results from the selected operating points have been normalized and combined into one universal plot, shown in FIG. 5, to mimic the behavior in a NEDC test with an optimized calibration for each fuel. A surprisingly large reduction in soot and NOx appears to be possible for the GTL diesel fuel and the EU50 and EU80 blends. These reductions are possible without hardware changes to the engine.

The neat GTL would allow for a simultaneous soot and NOx reduction of at least 35% compared to the EU diesel calibration. For constant engine-out soot emission, a NOx reduction of 45% seems possible. Due to the non-linear response with the GTL blends, reductions in soot and NOx that are greater than expected when considering the blending ratio, could be obtained with the EU80 and EU50 fuels. This non-linear response is depicted graphically in FIG. 6.

A 50% GTL blend would recover approximately 85% of the soot/NOx benefits of neat GTL, while a 20% GTL blend would recover approximately 48% of the benefit. It should be noted that the results shown so far have been facilitated by a simple and cost-efficient software adaptation only. It is to be expected that further improvements will be possible if additionally hardware changes, e.g. in the injection system and/or the combustion chamber design are taken into account. 

1. A method for the production of a diesel fuel composition, said method using Gas-to-Liquid (GTL) diesel fuel which has a density at 15 deg C. of below 0.78 kg/l, a sulphur content of less than 1 mg/kg, polyaromatics below 0.1 mass %, and a cetane number above 65, as a blend component together with at least a crude oil derived diesel fuel, which crude oil derived diesel fuel has a density at 15 deg C. below 0.85 kg/l, a sulphur content of less than 10 mg/kg, a polyaromatics content of below 5 mass %, and a cetane number from 51 to 60, said method producing a diesel fuel composition which, when combusted in an engine, has reduced NOx and soot emissions.
 2. A method as claimed in claim 1, wherein the reduction of the NOx and soot emissions are reduced in comparison to the crude derived diesel non-linearly to the blending ratio of the GTL diesel in the crude oil derived diesel fuel.
 3. A method as claimed in claim 1, wherein the reductions in both NOx and soot emissions for a 1:1 GTL:Crude derived diesel ratio are greater than 70% of the reduction obtained with a 100% GTL diesel fuel.
 4. A method as claimed in claim 1, wherein the reductions in both NOx and soot emissions for a 1:4 GTL:Crude derived diesel ratio are greater than 40% of the reduction obtained with a 100% GTL diesel fuel.
 5. A method as claimed in claim 1, wherein the GTL to crude oil derived diesel ratio is from 99:1 to 1:99 and the diesel fuel composition produced has a molar H:C ratio of between 1.8:1 and 2.1:1.
 6. A method as claimed in claim 5, wherein the molar H:C ratio is from 1.85:1 and 2.05:1.
 7. A method as claimed in claim 6, wherein the molar H:C ratio is from 1.9:1 and 2.00:1.
 8. A diesel fuel composition comprising both crude oil derived diesel fuel, which crude oil derived diesel fuel has a density at 15 deg C. below 0.85 kg/l, a sulphur content of less than 10 mg/kg, a polyaromatics content of below 5 mass %, and a cetane number from 51 to 60, and Gas-to-Liquids (GTL) diesel fuel, which GTL diesel has a density at 15 deg C. of below 0.78 kg/l, a sulphur content of less than 1 mg/kg, polyaromatics below 0.1 mass %, and a cetane number above 65, in a volumetric ratio range of from 1:99 to 99:1 and with a molar H:C ratio of between 1.8:1 and 2.1:1.
 9. A diesel fuel composition as claimed in claim 8, which diesel fuel composition has a density at 15° C. of from 0.77 kg/l to 0.84 kg/l.
 10. A diesel fuel composition as claimed in claim 9, which diesel fuel composition has a density at 15° C. of from about 0.8 kg/l to about 0.82 kg/l.
 11. A diesel fuel composition as claimed in claim 8, which diesel fuel composition has a lower heating value of from 42 500 kJ/kg to 43 800 kJ/kg.
 12. A diesel fuel composition as claimed in claim 11, which diesel fuel composition has a lower heating value of from 43 100 kJ/kg to 43 600 kJ/kg.
 13. A diesel fuel composition as claimed in claim 12, which diesel fuel composition has a lower heating value of from 43 200 kJ/kg to 43 500 kJ/k.
 14. A diesel fuel composition as claimed in claim 8, having less than 10 mg/kg sulphur.
 15. A diesel fuel composition as claimed in claim 8, having less than 5 mass % polycyclic aromatics.
 16. A method of operating a Cl engine to reduce NOx and soot emissions when compared to use of said engine with crude derived diesel fuel, said method including combusting a diesel fuel composition as claimed in claim 8 in the engine under said engine's operating conditions, wherein the reductions in both NOx and soot emissions for a 1:1 GTL:Crude derived diesel ratio being combusted in the engine are greater than 70% of the reduction obtained when a 100% GTL diesel fuel is combusted in the engine.
 17. A method as claimed in claim 16, wherein the reductions in both NOx and soot emissions for a 4:1 GTL:Crude derived diesel ratio being combusted in the engine are greater than 40% of the reduction obtained when a 100% GTL diesel fuel is combusted in the engine. 